Voltage regulators include an amplifier for generating a regulated voltage corresponding to the difference between a reference voltage input and a regulator feedback voltage. Also included are a power transistor which is driven by the amplifier and a bias network. The power transistor boosts the amplifier output to generate a regulated voltage output, which is fed back to the amplifier as the feedback voltage. The bias network sets the bias current in the amplifier based on one or more bias voltages generated by the network. Voltage regulators are at least partially disabled from time-to-time to reduce power consumption when load currents are low and steady, e.g., during low power or standby modes. When a voltage regulator is disabled, the amplifier bias current is substantially reduced to lower power consumption.
One conventional approach for disabling a voltage regulator is to set the gate-to-source voltage of the regulator power transistor to zero volts, thus turning off the power transistor. A switch may also prevent current flow through the bleeder resistor coupled to the power transistor. The regulator amplifier is also disabled by disconnecting the main bias voltage applied to the bias network, thus disabling the bias network. Each output node of the bias network is driven to an appropriate voltage level when the bias network is disabled to ensure that the amplifier is properly disabled. This way, the bias voltages applied to the amplifier do not float to problematic levels.
When the voltage regulator is subsequently re-enabled, the bias network charges the internal capacitance of the amplifier from a disabled state to a desired level before the amplifier can generate a properly regulated output. Some conventional voltage regulators include a startup circuit such as a boost capacitor network for assisting the bias network in setting the amplifier bias current during regulator re-enablement. The startup circuit helps in charging/discharging the bias voltages from their disabled levels to their proper operating levels.
However, conventional regulator startup circuits are highly process, voltage and temperature (PVT) dependent. For example, switch resistance and boost capacitance vary over process and temperature conditions. Also, the initial boost voltage provided by such circuits varies greatly with supply voltage. PVT-induced variations in startup circuit operation are conventionally unrelated to PVT-induced variations in bias network operation. That is, conventional startup circuits do not behave the same way as bias networks in response to varying PVT conditions. The regulator amplifier may not be properly enabled when the bias network and startup circuit behave differently under changing PVT conditions. The output of the regulator may fall outside acceptable limits required for proper circuit operation when the regulator is not properly enabled.